Recently, demand for lithium secondary batteries as power sources of personal digital assistants such as mobile phones, smart phones, tablet PCs and the like or electric automobiles such as hybrid electric cars, plug-in electric cars and the like has greatly increased. Particularly, high-power and high-energy-density active materials, which can replace cathode and anode materials of commonly-used lithium secondary batteries, have actively been developed.
Most of commonly-used lithium secondary batteries are problematic in that high-speed charge and discharge thereof is difficult because the theoretical capacity of graphite used as an anode is about 372 mAh/g and the interlayer diffusion speed of lithium is low. As active materials for overcoming this problem, silicon-based composite anode materials having a theoretical capacity of about 4200 mAh/g have attracted considerable attention for more than the last twenty years. Particularly, commercially-available silicon-graphite composite anode materials have been competitively developed in the related industry. However, these silicon-graphite composite anode materials reach a limit to the competition with graphite in terms of process cost in spite of their high energy density and improved charge-discharge cycle characteristics.
Meanwhile, like most metal materials electrochemically alloyed with lithium, silicon is also required to have its particles converted into nanosized particles and its performance to be improved by combination with lithium active/inactive materials in order to solve problems of mechanical damage of an electrode caused by volume expansion and contraction due to charge and discharge and the rapid reduction of the lifecycle of an electrode caused by mechanical damage thereof.
Most research into manufacturing a nanosized silicon-based anode is based on a mechanical pulverization method, a mechanical compounding method, a vapor synthesis method, a solution-based chemical synthesis method or the like. This nanosized silicon-based anode exhibits excellent characteristics as an anode of a secondary battery, but cannot be easily put to practical use as a commercially-available anode due to problems of complicated synthesis processes, high material cost, impurity influx, cost for waste disposal, formation of oxides accompanying synthesis procedure, and the like.
Electric explosion technologies have been developed over a long period of time as technologies for synthesizing nanopowder in large quantities. Recently, a technology for electrically exploding a semiconductor material in a liquid (Korean Patent Application No. 10-2008-0126028) was developed, thus proposing a possibility of silicon being electrically exploded in a liquid. However, since silicon is oxidized into SiO2 in an aqueous solution, there is a problem in that it is not suitably used as an anode active material for a lithium secondary battery. Further, since silicon is converted into a large amount of silicon carbide and is formed on the surface thereof with a carbon layer in an organic solvent such as ethanol, hexane or the like, there is a problem in that the performance of a silicon nanocomposite is greatly deteriorated.